CN1769317A - Method for preparing polylactic acid block copolymer using double screw rod extruder - Google Patents

Abstract

The invention provides a method for preparing polylactic acid block copolymer by using a twin-screw reaction extruder. The method uses polylactic acid prepolymer (HO-PLA-OH) with hydroxyl groups at both ends and polycaprolactone (OCN-PCL-NCO) with -NCO groups at both ends prepared by direct melt polycondensation method as raw materials. Copolymers of PLA and PCL prepolymers were prepared by a twin-screw reactive extruder. By optimizing the conditions, polylactic acid copolymerization products with high relative molecular weight and good performance can be obtained, and the viscosity-average molecular weight of the final product can reach 200,000. The method integrates the metering, transportation, mixing, reaction of raw materials and the processing of molten products, and has the characteristics of simple process, low production cost, large-scale continuous production and the like.

Description

Method for preparing polylactic acid block copolymer with twin-screw reaction extruder

technical field

The invention relates to a method for preparing high-molecular-weight polylactic acid block copolymers, in particular to a method for preparing polylactic acid block copolymers with a twin-screw reaction extruder, which can be used for continuous production of high-performance polylactic acid materials.

Background technique

Polylactic acid is a renewable biodegradable material with good biocompatibility. Because of its non-toxic, degradable, renewable and other characteristics, it meets the requirements of environmental protection and sustainable development, so it has attracted more and more attention from material researchers and business people. The glass transition temperature of high-molecular-weight polylactic acid is about 60°C, and the melting point is about 180°C. It is a thermoplastic polymer with similar physical and mechanical properties to polystyrene (PS) and polyester (PET), and its processing performance is good. It can be extruded, injected, and blow-molded using general-purpose plastic processing equipment, and can also be blended or copolymerized with general-purpose plastics. In addition, polylactic acid can also be spun into fibers by melt spinning, and its fiber fabrics have strong wrinkle resistance. Breathable and comfortable to wear. Therefore, as a new biodegradable material, polylactic acid has very broad application prospects.

Since the application of polylactic acid in many aspects requires high molecular weight, the preparation technology of low-cost high molecular weight polylactic acid has become a research hotspot in recent years. The currently widely used methods for preparing high molecular weight polylactic acid include direct polycondensation method, ring-opening polymerization method, copolymerization method and chain extension method. In the industry, the ring-opening polymerization method is mainly used. First, lactic acid is made into lactide, and then the lactide is ring-opened and polymerized. Cargill-Dow of the United States has built a 140,000-ton/year industrial production line using this method, and has applied for many patents. The biggest disadvantage of this method is that it needs to prepare lactide of polymerization grade purity, the process is complex and the cost is relatively high. The direct melt polycondensation method is the most economical method, but the polylactic acid synthesized by this method has a low molecular weight and has not yet been produced on an industrial scale. In order to increase the molecular weight of polylactic acid and improve its impact resistance, degradation resistance and other properties, many researchers have carried out research work on the copolymerization method. Patents such as US5631344, US5916998, and US5952913 describe the addition of comonomers during ring-opening polymerization to prepare high molecular weight copolymers. But these methods all adopt lactide as raw material, the cost is higher, and it is difficult to popularize and apply. Patents such as US5922832 use high-molecular-weight PLA to blend with other polymers to improve impact resistance, and the premise of its application is still the preparation of high-molecular-weight PLA. Recently, the present inventor has reported that the low molecular weight PLA obtained by the direct polycondensation method and the PCL of the -NCO group at both ends are copolymerized into high molecular weight PLA-PCL multi-block copolymers (Journal of Polymer Science: PartA: Polymer Chemistry, 2004, 42, 5045-5053), the synthesis of the block copolymer is mainly carried out in a three-necked flask, but in the process of preparing a copolymer in a three-necked flask, along with the polymerization reaction, the viscosity of the reaction system is getting higher and higher. High, which seriously hinders the continuation of the reaction, especially in the later stage of the reaction, the viscosity becomes larger, the mixing effect of the two components is not ideal, and the product performance becomes poor. Moreover, the preparation of polylactic acid block copolymers in a three-necked flask cannot achieve continuous production. Therefore, in order to realize the industrialization of the new synthetic method for preparing polylactic acid block copolymers established in this article, the inventors continue to explore new process routes . At present, there are few reports on the preparation of polylactic acid by twin-screw reaction extruder method, which is limited to the ring-opening polymerization and free radical grafting reaction of lactide. The present invention aims to provide a method for preparing high molecular weight polylactic acid block copolymers with low cost and easy industrial production.

Contents of the invention

The purpose of the present invention is to provide a method for preparing polylactic acid block copolymers with a twin-screw reaction extruder, which is a method for continuously producing high molecular weight and high-performance polylactic acid at low cost.

The purpose of the present invention is implemented like this: the polylactic acid prepolymer with the hydroxyl group at both ends and the polycaprolactone prepolymer with -NCO group synthesized at both ends in advance are added twin-screw reactor, control two kinds of materials Adjust the feed amount, adjust the screw temperature and screw speed, and after the reaction is extruded, the melt is directly cooled by a water bath to form strips. The polylactic acid prepolymer with hydroxyl groups at both ends of the molecule is obtained by direct dehydration polycondensation of lactic acid monomers and glycols such as polyethylene glycol, and its relative molecular mass is 0.3 to 25,000; The polycaprolactone prepolymer of the group is prepared by reacting polycaprolactone and diisocyanate, and its relative molecular mass is 0.1-10,000; the above two kinds of prepolymers are synthesized by conventional known methods.

The specific implementation is as follows: the previously synthesized polylactic acid prepolymer with hydroxyl groups at both ends and the polycaprolactone prepolymer with -NCO groups at both ends of the molecule are respectively added to the twin-screw reactor through a feeder and a metering pump, and the Adjust the frequency of the feeder and the metering pump to control the feeding amount of the two materials. The weight ratio of the polylactic acid prepolymer with hydroxyl groups at both ends and the polycaprolactone prepolymer with -NCO groups at both ends is 1-9:1; preferably 5-1.5:1. The screw temperature is 100-200°C, the screw speed and the screw frequency are 1-5Hz. After the reaction is extruded, the melt is directly cooled in a water bath to form strips and pellets.

The reaction formula is as follows:

Among them, m+n=30~340; m'+n'=10~50.

The viscosity-average molecular weight of the polylactic acid block copolymer is 80,000 to 200,000.

The twin-screw reactive extruder provided by the invention is characterized in that the polylactic acid block copolymer method is prepared:

1. The metering, transportation, mixing, reaction of raw materials and the processing of molten products are integrated, so as to realize continuous production, simplify the process flow and reduce the production cost.

2. The extrusion process can be adjusted at any time to adapt to different material systems, and can produce various types of products.

3. The twin-screw reaction extruder has a better mixing effect than the tank reactor, and the resulting product has better performance.

4. Compared with ordinary equipment, the twin-screw extruder as a chemical reactor can greatly reduce the reaction time.

Description of drawings

Figure 1. Infrared spectrum.

In accompanying drawing 1, (a) is OCN-PCL-NCO, (b) is HO-PLA-OH, (c) is the blend of HO-PLA-OH and OCN-PCL-NCO, (d) is PLA - PCL multi-block copolymers.

Detailed ways

The following examples help to understand the present invention, but are not limited to the content of the present invention:

Example 1:

The twin-screw parameters are set as follows: The temperature of each zone of the screw (°C) Screw Frequency(Hz) Zone I Zone II Zone III Zone IV nose 120 160 170 165 180 4

Feed the polylactic acid prepolymer PLA with hydroxyl groups at both ends and the polycaprolactone prepolymer PCL with -NCO groups at both ends of the molecule in a weight ratio of 70/30, twin-screw extrusion, cooling, and pelletizing to obtain poly Lactic acid multi-block copolymer. Fig. 1 is the infrared spectrogram of the polylactic acid multi-block copolymer prepared in this embodiment. From the change of the characteristic absorption peak of -NCO end group at 2270cm ^-1 , it can be proved that the PLA prepolymer and the PCL prepolymer have reacted during the twin-screw extrusion process, and PLA can be obtained by this twin-screw reactive extrusion method. and PCL block copolymers. The viscosity-average molecular weight of the obtained polylactic acid block copolymer measured by the viscosity method is 100,000.

Example 2

PLA and PCL block copolymers were prepared according to the method described in Example 1. The screw parameters are kept constant, and the weight ratio of PLA and PCL prepolymer is fed according to the ratio of 60/40, twin-screw extrusion, cooling, and pelletizing to obtain a polylactic acid block copolymer. The viscosity-average molecular weight of the prepared copolymer measured by the viscosity method is 190,000.

Example 3

PLA and PCL block copolymer were prepared according to the method described in Example 1. The screw takes the following parameters: The temperature of each zone of the screw (°C) Screw Frequency(Hz) Zone I Zone II Zone III Zone IV nose 130 160 165 170 200 3

The feeding weight ratio of PLA and PCL prepolymer is 70/30, twin-screw extrusion, cooling, pelletizing, to obtain polylactic acid block copolymer. The viscosity-average molecular weight of the prepared copolymer measured by the viscosity method is 180,000.

Example 4

PLA and PCL block copolymers were prepared according to the method described in Example 1. Screw rod adopts the parameter of embodiment 3. The feeding weight ratio of PLA and PCL prepolymer is 60/40, twin-screw extrusion, cooling, pelletizing, to obtain polylactic acid block copolymer. The viscosity-average molecular weight of the prepared copolymer measured by the viscosity method is 200,000.

Example 5

PLA and PCL block copolymers were prepared according to the method described in Example 1. Screw rod adopts the parameter of embodiment 3. PLA and PCL prepolymer feeding weight ratio is 80/20, twin-screw extrusion, cooling, pelletizing, to obtain polylactic acid block copolymer. The viscosity-average molecular weight of the prepared copolymer measured by the viscosity method is 80,000.

Claims (6)

1. A method for preparing polylactic acid block copolymers with a twin-screw reaction extruder, characterized in that the polylactic acid prepolymer with hydroxyl groups at both ends of the molecule and the polycaprolactone with the NCO group at both ends of the molecule The prepolymer is used as a raw material, mixed, heated and extruded on a twin-screw reaction extruder, cooled into strips and pelletized; wherein the weight ratio of the polylactic acid prepolymer and polycaprolactone prepolymer is: 1-9 : 1. 2. method as claimed in claim 1, it is characterized in that the polylactic acid prepolymer with hydroxyl at both ends of the molecule is made by direct dehydration polycondensation of dibasic alcohols such as lactic acid monomer and polyethylene glycol, and its relative molecular mass is 0.3-25,000; the polycaprolactone prepolymer with -NCO groups at both ends of the molecule is prepared by the reaction of polycaprolactone and diisocyanate, and its relative molecular mass is 0.1-10,000. 3. The method according to claim 1 or 2, characterized in that the viscosity-average molecular weight of the polylactic acid block copolymer is 80,000 to 200,000. 4. The method according to claim 1, characterized in that the twin-screw reactive extruder used is a reactive twin-screw, and the residence time of the material in the screw is 4 to 12 minutes. 5. The method according to claim 1 or 2, characterized in that the polylactic acid prepolymer and the polycaprolactone prepolymer are added to the twin-screw reactor through a feeder and a metering pump respectively. 6. The method according to claim 1 or 4, characterized in that the temperature of the screw is 100-200° C., and the frequency is 1-5 Hz.

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